Transmission of power over long distances can advantageously be performed using HVDC transmission lines. In an AC transmission system, the transmission losses are dependent on both active and reactive power transfers. For long transmission lines, the losses due to the reactive power transfer will be significant. In an HVDC transmission system, on the other hand, only active power is transferred. The losses in an HVDC transmission line will thus be lower than the losses in an AC transmission line of the same length. For long distance transmission, the higher investment of necessary conversion equipment in an HVDC system is often justified.
Most HVDC transmission systems in use today are point-to-point transmission systems, where electric power is transmitted from one AC system to another. This is an efficient way of transmitting electrical power to/from remote areas, across water, between two unsynchronized AC grids, etc. In many circumstances, however, multi-point HVDC transmission systems, where power can be transmitted to/from at least three different points in one or several AC networks, are desired. A multi-point HVDC transmission system will here be referred to as a DC grid. One example of when a DC grid can be useful is when connecting (multiple) off-shore wind farms to (multiple) on-shore power grids. Another example is when transferring large amounts of power over long distances in existing AC grids, in which case low loss transmission can be achieved by using a DC grid as a backbone or over-lay grid to the existing AC grids.
A drawback of DC transmission as compared to AC transmission is that the interruption of a fault current is more difficult. A fault current in an AC system inherently exhibits frequent zero crossings, which facilitate for fast current interruption. In a DC system, no inherent zero crossings occur. In order to break a DC current, a zero crossing of the DC current generally has to be forced upon the system.
Moreover, in an AC system, the fault current will be limited by the reactance of the transmission lines. In a DC system on the other hand, the inductance of a transmission line will only matter in the transient stage. When the transient (quite quickly) diminishes, only the resistance of the lines will limit the level of the fault current on the DC side. Thus, the fault current can grow very rapidly in a DC grid. A fast breaking of a fault current is therefore desired.
Furthermore, power from the AC side will be fed into a fault that occurs on the DC side. Typically, this implies that the fault currents are high on the DC side, whereas the DC voltages are low throughout the DC grid, making organized power transfer impossible during the faulted time period. This is particularly pronounced when at least some of the converters are based on Voltage Source Converter (VSC) technology, since the switches of a VSC converter will typically have to be blocked when the current rises above a certain level, leaving the VSC converter basically operating as a diode bridge. The more converters that are connected to the DC grid, the higher the DC current in the fault. The situation of having depressed DC voltages, with the consequential power transfer inability, may, if prolonged, have serious impact on the AC system stability. AC system instability would result in black-outs, which are very costly for society. In order to prevent AC system instability, the AC systems could be designed with substantial reserve transfer capability. However, such over-dimensioning of the AC systems is very costly and generally not desired. Hence, a fast breaking of a DC fault current, before the DC voltages have collapsed, is desired.
Thus, in order to limit the effects of a line fault, an HVDC breaker should react very fast, typically in the transient stage while the fault current still is increasing and before the DC voltages have collapsed too much. Efforts have been put into the development of fast and reliable HVDC breakers, and the HVDC breakers that currently provide the fastest interruption of current are based on semi-conducting technology. A semi-conductor HVDC breaker is for example disclosed in EP0867998. However, semi-conductor HVDC breakers experience a power loss which is higher than in a mechanical breaker. Furthermore, semi-conductor HVDC breakers designed to break large currents are considerably more expensive than mechanical breakers. However, existing mechanical breakers cannot provide sufficient breaking speed. Thus, there is a need for cost- and energy effective fault current handling in a DC grid.